Field of the Invention
The present invention relates to a zoom lens and an imaging apparatus including the zoom lens. The present invention is suitable for an imaging apparatus with an image pickup element, such as a digital still camera, a video camera, a monitoring camera, and a broadcasting camera, and an imaging apparatus such as a camera with a silver-halide photographic film.
Description of the Related Art
Imaging apparatuses, such as a digital still camera and a video camera, that use a solid-state image pickup element have been improved to a point that the size of the entire apparatus continues to decrease while its functionality increases. To that end, a zoom lens used in these apparatuses is required to be compact in size, have a high magnification, and provide high optical performance. To meet these requirements, there has been known a zoom lens including lens units that respectively have, in order from an object side to an image side, positive refractive power, negative refractive power, positive refractive power, and positive refractive power.
In zoom lenses of Japanese Patent Application Laid-Open No. 11-23965 and Japanese Patent Application Laid-Open No. 2008-310222, zooming is performed by moving a second lens unit. In these zoom lenses, focusing and compensation of image plane variations caused by zooming are performed by moving a fourth lens unit.
A zoom lens that performs focusing by moving lenses provided on an image side of a first lens unit in this manner is referred to as an inner focus zoom lens. In general, in an inner focus zoom lens, an effective diameter of a first lens unit becomes smaller and the lens system can be more compact, as compared to a zoom lens that performs focusing by moving its first lens unit.
Generally, in a zoom lens, reducing the number of lenses while enhancing the refractive power of each lens unit of the zoom lens can downsize the entire system while achieving a high magnification. Nevertheless, when the refractive power of each lens surface becomes strong, reduction in optical performance is likely to occur.
In a zoom lens including lens units respectively having, in order from an object side to an image side, positive refractive power, negative refractive power, positive refractive power, and positive refractive power, it is important to appropriately set refractive power of each lens unit and materials of the lenses to maintain high optical performance while realizing a higher magnification and a further downsizing the lens' size.